Pirate King level is Akainu’s tier, remember Akainu can complete the manga in less than a year according to Oda.
Meanwhile Kaido can only rival characters who were defeated by Ace according to Oda. Lol
If I recall correctly when Oda said that he was talking about why he gave Luffy the Gomu Gomu
He said something a goofy non-overpowered power like that will make the battles more challenging and interesting to read
If he gave Luffy something overpowered like the magma fruit instead One Piece would have ended in one year. Because he would just magma fist everything and there would be no tension lol
Point is the magma fruit is overpowered, not that Akainu is top 1 in the verse
Btw Akainu probably improved since Marineford, let's say for argument sake that he is on the level of Prime Roger (or Garp). Even if that's the case, so what? With the recent power creep, Imu shits on everybody we ever thought was powerful. And that includes Roger, Whitebeard, Akainu, you name it.